Key Ideas

Ideas preceded by an asterisk (*) are discussed in the Boxes.

Constellations and the Celestial Sphere: It is convenient to imagine the stars fixed to the celestial sphere with Earth at its center.

Diurnal (Daily) Motion of the Celestial Sphere: The celestial sphere appears to rotate around Earth once in each 24-hour period. In fact, it is actually Earth that is rotating.

Seasons and the Tilt of Earth’s Axis: Earth’s axis of rotation is tilted at an angle of about 23½° from the perpendicular to the plane of Earth’s orbit.

Precession: The orientation of Earth’s axis of rotation changes slowly, a phenomenon called precession.

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Timekeeping: Astronomers use several different means of keeping time.

The Calendar: The tropical year is the period between two passages of the Sun across the vernal equinox. Leap year corrections are needed because the tropical year is not exactly 365 days. The sidereal year is the actual orbital period of Earth.